Construction of Scotland’s biggest infrastructure project, the Forth Estuary’s new Queensferry Crossing, grows more spectacular daily.
Roadworks at both ends of Scotland’s Forth Road Bridge appear the main reason for a further proposed speed limit reduction of 20% to 40mph this summer. But few motorists will realise that an additional safety concern lies not on the road itself but alongside, in the highly visible eye-catching construction work underway in the middle of the Forth Estuary.
Here - just a few hundred metres up river - towers, decks, cranes and falsework for the estuary’s third crossing are now rising higher from the water by the day, as the UK’s largest ongoing bridge project becomes increasingly recognisable.
All three concrete towers for the £790M cable-stayed Queensferry Crossing are growing, on average, 4m taller every week and now stand over two thirds their eventual 210m height. From 60m up each tower, short deck sections both sides will soon start cantilevering outward across the estuary, extending 32m for each new pair erected.
“This is our busiest and most exciting construction year,” says Michael Martin, project director for contractor Forth Crossing Bridge Constructors (FCBC), a joint venture of Hochtief, Dragados, American Bridge International and Morrison Construction.
“It’s great to be finally out of the water, with our main construction risks behind us, and see the structures grow upward and outward.”
Protruding just above the waterline, the tops of temporary sheet pile walls for the towers’ massive caisson foundations are the sole reminder of those underwater risks that challenged the construction team for much of the last two years. But, with these 30m deep concrete cylinders sitting securely on seabed rock, engineers are optimistic that the greatest risk now is the exposed Forth Estuary’s unpredictable weather.
High winds, gusting to over 40m/s at the tower tops and whipping up a constantly choppy sea, will this year continue to dominate the construction programme.
Last autumn, the most expensive vessel in the contractor’s 30 strong marine fleet - the 1,200t capacity Taklift 6 floating shearleg crane - was at the mercy of winds and waves for two months as it completed 30 massive lifts. A wind speed above 12m/s would trigger downtime as this giant crane lifted 60m high steel falsework from floating barges and secured it to the towers.
Needed to support initial deck sections, these 146t raking triangular trusses now straddle both sides of each tower and are topped by temporary steel platforms. This total 900t mass of falsework had to be positioned to tolerances of just 50mm.
“We reverse engineered much of the falsework design specifically to match the crane’s lifting characteristics,” recalls FCBC section manager for the deck Jonathan Davies. “But at least the wind was less severe than we feared during the 60 day weather window we needed for the Taklift operation.”
Onto these falsework platforms, now secured around the three towers, Taklift 6 has lifted the first 12, 29.5m wide steel deck units. Two of these 16.2m long trapezoidal box section units are now welded together on both sides of each tower.
Their initial role is to support the project’s half dozen deck erection traveller hoists - also positioned by the Taklift. Late this spring, the hoists will start lifting the bridge’s main balanced-cantilever deck sections extending out both sides of each tower.
All 110 of these 4.5m deep deck boxes are now being preassembled into 40m wide composite steel and concrete sections at the contractor’s quayside storage yard in Rosyth Docks, 3km upriver from the bridge site.
Here the steel boxes weighing an average of 250t and shipped from a Shanghai fabrication factory, are having their maximum 460mm thick reinforced concrete deck poured before they are barged to the bridge site.
As the traveller hoists start lifting the arriving 720t completed deck sections, they will work in sequence each side of a tower to create the balanced cantilevers. Unusually, this operation involves just one erection traveller on each side, with its lifting cables attached to two centrally located points on the barged-out deck unit beneath. Normally there would be two hoists working in tandem, using lift points each side of the deck unit being raised.
The reason is the overall cable-stayed deck design, which calls for support stays to run centrally down the spine of the bridge, fixed into the carriageway’s central reserve.
Over the next 12 months, Davies and his team will create what is understood to be the world’s largest three-tower, cable-stayed deck. And also, from the central tower, they will form what is likely to be the longest single balanced-cantilever deck length totalling 650m. The bridge, including approach viaducts, will stretch 2.7km.
Originally only four traveller hoists were to be used on the entire project, erecting balanced cantilevers from two of the towers simultaneously. One pair would then move across to work from the third tower.
But, soon after the JV won its six year design and build contract back in 2011, it opted for an accelerated plan of employing six travellers simultaneously.
“It’s great to be finally out of the water, with our main construction risks behind us, and see the structures grow upward and outward”
Michael Martin, FCBC
FCBC’s Jaime-Wilson Castro, the Colombian engineer in charge of tower construction, hopes to finally hand over responsibility for the construction programme’s critical path to those overseeing the balanced cantilever deck early this summer.
The three concrete towers - roughly rectangular in section, though with two sides curved - are being built simultaneously. Since September 2013, their strip and jump climbing formwork platforms have been creating, on average, one 4m lift every week and all towers now stand over 140m high.
Castro’s critical-path handover will be reached soon after he starts casting into the towers’ vast 40t anchor boxes for the bridge’s cable stays. Fabricated in China, each of these 4m high steel boxes contains, in general, four anchor tubes each side from which the cables will run down to deck sections as they are erected.
By late spring the insertion rate of these anchor boxes, starting at a tower height of around 150m, should begin to overtake deck erection, transferring the project’s time-critical construction operation down to the carriageway.
“High winds remain our major risk and affect tower-top crane operations”
Jaime-Wilson Castro, FCBC
“High winds remain our major risk and affect tower-top crane operations,” says Castro. “But even when these cranes cannot service us, there is still a lot we can do inside our well protected work platforms.
Known affectionately as “birdcages”, each four-level, 11m tall concreting platform is totally enclosed. Plywood-faced steel-framed shutters, for the tower’s four sides, hang from the birdcages’ upper levels with electrically powered jacks raising them in a single 4m stroke.
Virtually all 54 pours per tower have a slightly different profile as the hollow columns taper from a 16m wide base with 1.6m walls, to just 5m at the top and walls 400mm thinner.
This profiling calls for totally adjustable formwork and for slightly different reinforcement cages every lift. And it is the simple but clever time-saving engineering solution the contractor has applied to rebar fixing that accounts for much of Castro’s low-stress work mode.
Every rebar cage is preassembled at FCBC’s Rosyth Dock storage yard from four large interconnecting flat panels. On the quayside an adjustable steel template creates each tower pour’s wall geometry, allowing its four bespoke, 4m tall rebar panels to be temporarily intertwined into position. By the time the panels are hoisted into place by the tower crane, their exact position - relative to the other three sides - is known.
“All the hard work is done in calm conditions down on dry land”
Jaime-Wilson Castro, FCBC
“All the hard work is done in calm conditions down on dry land,” claims Castro. “Given no crane downtime potential, we can sometimes cut our planned one week concreting cycle down to just five days.
“Apart from prefabricating the rebar cages, jump-form construction of the towers is conventional,” he points out. “The last thing you want is to introduce innovation 200m up in the air.”
FCBC’s concrete batching plant in Rosyth docks will provide all 500,000t needed for the entire project, including tower walls. Barged to the bridge site in static mixing drums, concrete for the towers is pumped up a 125mm diameter static line and placed conventionally.
Tower concrete design is a balance between extended workability, good flowability and high early strength, allowing shutter striking at 36 hours. A low 0.36 water/cement ratio and 60% cement replacement by blast furnace slag, is proving the ideal mix and Castro is on schedule for his last tower pour this summer.
Client Transport Scotland remains equally confident and confirms overall construction is also on schedule for bridge completion by Christmas next year.
“We still face risks from the weather, but I am reasonably relaxed over the work still to be done,” says Transport Scotland project director David Climie. “Now we are well above the water, construction involves only proven and tested technology.”
Climie is also relaxed over total costs. The £790M main bridge and approach roads contract is, for the client, financially secure, as all risks - bar inflation - are carried by the contractor.
And with the accumulative inflation figure since December 2010 running at half the client’s lowest estimate, Climie can already offer the Scottish Parliament around £195M savings over original projected project costs of up to £1.6bn.
Robbed of the deep water essential for marine-based erection, the 765m of side spans are being formed by incremental launching. Bridge geometry does though dictate a different technique at either side.
On the southern bank, the crossing’s two carriageways are split, each demanding separate single trapezoidal box deck units. The carriageways are formed of 28.4m long box sections welded together in a riverside assembly area into launch sections which are 90m long on average.
Pairs of 500t strand jacks, secured to concrete bankside abutments, pull each launch section out into the estuary over six V-shaped concrete piers at a rate of roughly 6m/h.
Each launch takes up to two days and 10 of the total 12 launches have been completed with the twin decks stretching 450m out from their abutments. The final 90m launches, to connect into central bridge spans, are planned for early summer.
These will pull a total deck weight of 3,100t and, with an up to 90m free cantilever, which could cause leading edge deflections of 440mm.
To prevent this, temporary 35m high “king post” towers are erected on the viaduct decks, some 85m back from their leading edges.
These resist leading edge deflection by using cables stays stressed to 450t and strung between deck leading edges and king post tower tops.
Two and a half kilometres away across the Forth Estuary, construction of the shorter northern approach viaduct will involve just a single 222m incremental launch. Scheduled for this autumn, similar jacking and king post arrangements will be used, but deck geometry - with single and then full-width double-carriageways needed - will trigger an even more complex launch sequence.
Both the single and double box deck units are now being welded together on site into one launch length, protected by a large tent. Near its riverside entrance lie, on the route of the launch, a pair of parallel concrete walls with tops formed as 2 degree downward-sloping ramps.
As this long single 5500t launch takes place its leading edge would start to deflect a maximum 2m downward. To counteract this, the two ramp-topped walls will play a vital role.
To help minimise the deflection, the tail end of the deck still on the bankside is lowered during the final stages of the launch by routing it across the two downward-sloping ramps. This forces forward deck sections to rise.
An intermediary river pier acts as a support and fulcrum as the moving deck passes over it.
By the time this forced upward movement reaches the launch’s leading edge and the king posts, the deck will have been raised the required 2m. This should exactly cancel out any natural deflection and connect, to 5mm accuracy, into the waiting free end of the main bridge cantilever.
“This simple but innovative technique will trigger a complex combination of load and deflections,” says FCBC’s construction director Marcos Gonzalez. “
It will involve interaction between temporary and permanent works but we are very confident it will work effectively.”